16f6c0aecf
Inline adoc files when they are included exactly once. Also fix language tags because this inlining gives us better information on what language the code is written in.
36 lines
1.3 KiB
Plaintext
36 lines
1.3 KiB
Plaintext
== Why is this an issue?
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``++UNION++`` is significantly less performant compared to ``++UNION ALL++`` because it removes duplicated entries and run an internal ``++DISTINCT++`` to achieve this.
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``++UNION ALL++`` is not removing duplicates and returns all the results from the queries. It performs faster in most of the cases compared to ``++UNION++``. Nevertheless, the quantity of data returned by ``++UNION ALL++`` can be significantly more important than with ``++UNION++``. On slow network, the performance gain to use ``++UNION ALL++`` instead of ``++UNION++`` can be challenged by the time lost to transfer more data than with ``++UNION++``.
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=== Noncompliant code example
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[source,sql]
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----
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-- if you care about not having duplicated entries, then UNION is the good choice
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SELECT EMAIL FROM EMPLOYEES UNION SELECT EMAIL FROM CUSTOMERS
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----
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=== Compliant solution
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[source,sql]
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----
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-- if you don't care about duplicated entries in the results of this UNION, then UNION ALL should be preferred
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SELECT EMAIL FROM EMPLOYEES UNION ALL SELECT EMAIL FROM CUSTOMERS
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----
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ifdef::env-github,rspecator-view[]
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'''
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== Comments And Links
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(visible only on this page)
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=== on 23 Oct 2018, 10:36:07 Alexandre Gigleux wrote:
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Merged with RSPEC-1571 which was better described to cover the case of ``++UNION ALL++``.
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endif::env-github,rspecator-view[]
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